Competitive Exams: Current Affairs 2012: Polio Eradication

Today, India passes one whole year without polio caused by natural (wild) poliovirus-a major milestone towards polio eradication. Many experts believed that India posed the greatest challenge to polio eradication for epidemiological reasons; our success proves it can be achieved in other countries where the obstacles are more programmatic than biological. For the Global

Polio Eradication Initiative (GPEI), this is a shot in the arm.

The year 2000 was the target date for global eradication set by the World Health Assembly in 1988. Intense efforts by countries, guided by GPEI, resulted in success in most countries and partial success in all countries.

Problems & innovative solutions

By 1988, nearly 70 countries had achieved the elimination of wild poliovirus transmission through their routine national immunisation programmes, some using the inactivated poliovirus vaccine (IPV) but others using the oral poliovirus vaccine (OPV).

For countries with polio, the World Health Organisation recommended the exclusive use of OPV for its low cost and ease of inoculation by mouth-as two drops. On the flip side, the very fact that many countries using OPV could not control polio with routine immunisation indicated that it was not as effective as in other countries. The difference was clear: Tropical/subtropical countries with low income, overcrowding, high birth rates, and high child mortality faced low effectiveness of OPV, whereas those with the opposites had high vaccine effectiveness.

In India, the disparities of such factors spelt differential effectiveness among States. Not only did some communities exhibit lower vaccine effectiveness, they also had more intense wild virus transmission.

The conjunction of both problems made U. P. And Bihar stand out as the most difficult regions for polio eradication. Wild polioviruses exist in 3 types, and OPV also contains attenuated strains of the 3 types. So it is called trivalent OPV (tOPV). Among the 3 types, type 2 is the most efficient; that was why type 2 wild virus disappeared in 1999, within a few years of national pulse vaccinations. But type 2 in the tOPV also interferes with the others, making them very inefficient.

While the problem of ‘failure of vaccine’ was being addressed, there was also the problem of ‘failure to vaccinate’ Seasonally, millions of families from UP and Bihar migrate for work-some to Maharashtra or Punjab, others within their States. Their children missed both routine and campaign doses. The tactic of vaccination in transit-in trains/buses and in stone quarries/brick kilns-became the norm from 2005. As all bottlenecks were cleared, success ensued.

For certification of eradication, two more years should pass without any case of wild virus polio. Poliovirus can remain silently in circulation for short periods; so, complacency must not set in. We must continue working as if we still have poliovirus lurking somewhere, only to show up when least expected. There is also the threat of importation of wild virus from Pakistan, Afghanistan and Nigeria.

Vaccine viruses by themselves can rarely cause polio; the balance is roughly one case of vaccine-associated paralytic polio (VAPP) replacing 200 cases of wild virus polio. Yet, in the absence of wild virus polio, VAPP is unacceptable. Moreover, vaccine viruses may gradually revert to wild-like properties if allowed to circulate. Such circulating vaccine-derived polioviruses (cVDPV) cropped up in many OPV-using countries recently, including India since 2009. If allowed to grow, they can capture the niche vacated by wild viruses. We have to stop OPV to stop VAPP, but some cVDPV may already be in silent circulation to show up in outbreaks one or more years later. The safest solution is to introduce IPV, reach 90 per cent or more coverage and only then stop OPV. That will pre-empt the evolution of cVDPVs. Only after we ensure the absence of wild and vaccine polioviruses in the population can we claim complete success of polio eradication. That is the challenge of the present decade.